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People who choose organic fruits and vegetables to avoid pesticides and other chemicals may have another reason to buy organic. A new study finds that organically grown tomatoes have higher levels of flavonoids, which may protect against cardiovascular disease.

Writing in The Journal of Agriculture and Food Chemistry, researchers said the level of one flavonoid in the organic tomatoes was almost twice as high as that in conventionally grown tomatoes.

Because of evidence that flavonoids may fight age-related diseases, the study said, researchers have been trying to develop crops with higher levels of them. In the United States, only potatoes are eaten more often than tomatoes.

The researchers, from the University of California, Davis, looked at tomatoes grown over a 10-year period in organic fields and regular ones. Not only did the organic tomatoes score better, they said, but over time their flavonoid levels kept increasing.

The lead author of the study, Alyson E. Mitchell, said she was surprised at the extent of the difference.

“We sort of went into this expecting higher levels,” Dr. Mitchell said. “We did not expect to find the levels that we found.”

The study offered several possible explanations, most having to do with the fertility of the soil. Organic farms, the researchers said, gradually improve the soil by letting organic matter accumulate through the use of cover crops, compost and manure.

The study also said flavonoids were among a group of metabolites produced by plants in part to ward off pests. So it is possible, the researchers said, that the increased pressure on organic crops from pests may result in more flavonoids.